Pellucid Argent OIII imaging filter 31 mm, observatory grade

Specifications

Description

The Argent OIII imaging filter is a narrowband interference filter in a 31 mm unmounted format, designed for a filter holder or drop-in slot in a camera or imaging train rather than a threaded eyepiece cell. Its passband is about 7 nm wide and centered near 500 nm, covering the doubly ionized oxygen emission lines at 495.9 nm and 500.7 nm. Peak transmission within that band is about 93 percent. Outside the passband, the coating blocks most of the visible spectrum, including much of the broadband glow from artificial lighting.

Emission nebulae concentrate their light into a few narrow lines, while the sky background and most stars radiate across a continuous spectrum. A narrowband filter passes the nebular line and rejects most of the background, which raises the contrast of faint nebular structure in long exposures. Under light-polluted skies the effect is larger, because blocked sky glow is a major source of background signal. The filter does not brighten the nebula; it lowers the background that the nebula's light must compete against.

Good targets include the Veil Nebula, the Cygnus Loop, the Dumbbell Nebula, and the Ring Nebula, all of which show strong oxygen emission. Galaxies, star clusters, and reflection nebulae emit little at these wavelengths, so the filter will dim them substantially and is not useful for those objects. Exposure times typically need to be increased to compensate for the reduced light, and focus may shift slightly compared with unfiltered imaging because the added glass changes the optical path.

Because the passband is narrow, the filter is sensitive to the angle of incidence. In faster beams the passband shifts toward shorter wavelengths, so the filter performs best in slower optical systems. Handle the glass by its edges, keep the coated surface clean, and store it in its sleeve when not in use.

About this configuration

All configurations of the Pellucid Argent OIII imaging filter 31 mm share the same filter glass design; the options change the grade, the coating, and the case. The observatory grade comes with a measured transmission curve for the individual filter in the box. Observatory grade glass has the tightest flatness tolerance in the Argent line, which suits imaging with long focal lengths. A standard coating suits visual use, where small reflections between filter and eyepiece rarely show.

A plastic case is light and stacks with other filter cases in an accessory box. The 2023 edition uses the same coating design as later years, with small differences in the cell markings. A filter does not make objects brighter; it darkens the sky background more than the target, which raises contrast.

What's in the box

• OIII filter, 31 mm unmounted
• Padded storage sleeve
• Test report with measured transmission curve
• Plastic storage case

Compatibility notes

• Fits a 31 mm unmounted filter holder or drop-in slot in compatible camera and imaging systems.
• Not threaded; it does not fit 1.25 in or 2 in eyepieces or nosepieces without a holder.
• Performs best in beams slower than about f/5, where the passband shift is smaller.